Moto lab builds 4-Mbit 'nanocrystal' memory

Austin, Texas - Using conventional silicon deposition techniques, Motorola engineers have built a 4-Mbit nonvolatile memory based on small islands of silicon measuring 50 angstroms in diameter.

The silicon nanocrystals could form the basis for a new type of nonvolatile memory that can be programmed and erased with lower voltages than existing varieties, said Ko-min Chang, director of the embedded-memory design group at Motorola's Semiconductor Products Sector.

The approach is part of a class of nonvolatile memories known as thin-film storage, which includes Motorola's Sonos flash development effort (see Nov. 25, 2002, page 27). The goal is to store a charge in a layer of nitride, or in the isolated silicon nanocrystals, between two layers of oxide.

First Sonos devices, then nanocrystal-based nonvolatile memories, could come to market in a few years as replacements for today's flash, Chang said.

Traditional floating-gate flash is difficult to scale. The dielectric material needed to form the capacitor is running into physical limits. And because relatively high voltages-12 V-are required to program bits through the floating gate, many of the transistors in a floating-gate flash memory must be relatively large. Scaling the voltage level down to plus/minus 6 V would help reduce the die size and cut power consumption, Chang said.

"Our initial target is to move to 6 V for programming, but we think with this [thin-film storage] approach we can go to even lower voltages," he said.

More reliableThe silicon nanocrystals could be more reliable than floating-gate flash memories, said Joe Mogab, vice president of Motorola's Advanced Products R&D Lab here. If a defect occurs in the continuous layer of polysilicon in floating-gate flash, all of the charge dissipates, creating major reliability and yield issues as scaling progresses.

Bruce White, a deposition engineer at the lab, said he has been working for the past several years on finding the right deposition chemistry, at the right time and temperature conditions, to control the nanocrystal formation. If the crystals are too small or dispersed, they will not hold sufficient charge. If they are too large or dense, the electrons may move to neighboring nanocrystals or leak through defects in the tunnel oxide underneath.

About 500 to 700 nanocrystals are needed to store one bit. Defects in a few of the silicon dots would not affect their ability to store or read charge. The silicon islands are spaced about 50 angstroms apart, and at that size quantum-mechanical forces tend to keep the electrons securely in their separate islands.

"The next step is to productize this technique and I don't see any problems" to that, White said.